Arc welding method

ABSTRACT

An arc welding method is described where pulse arc welding is performed with a welding wire being fed in a forward direction during a first period and short-circuit transfer arc welding is performed with the welding wire being fed in the forward direction and a reverse direction during a second period. The arc welding method alternately switches between the first period and the second period, where the switching of the first period to the second period is performed during a peak period of the pulse arc welding.

FIELD

The present disclosure relates to an arc welding method in which weldingis performed by alternately switching between two kinds of periods,i.e., a period for pulse arc welding and a period for short-circuittransfer arc welding.

BACKGROUND

A conventional welding method may include the step of feeding a weldingwire and the step of alternately switching between a period in whichpulse arc welding is performed and a period in which short-circuittransfer arc welding is performed (see JP-A-2005-313179, for example).The switching frequency of the two periods may be approximately 0.1 to10 Hz. Such a method can produce scale-like beads with a goodappearance. In addition, the method can control heat input to the basematerial by adjusting the ratio between the pulse arc welding period andthe short-circuit transfer arc welding period.

JP-A-2015-205347 discloses another arc welding method in which thewelding wire is fed in a forward direction during a pulse arc weldingperiod, while it is fed in the forward and reverse directionsalternately during a short-circuit transfer arc welding period. Morespecifically, the short-circuit transfer arc welding period includes anarc period and a short-circuit period, and the welding wire is fed inthe forward direction during the arc period, while being fed in thereverse direction during the short-circuit period. In addition,according to the conventional method, the pulse arc welding is switchedto the short-circuit transfer arc welding during a base period thatbegins after a droplet of molten wire is transferred by the pulse arcwelding.

In the conventional techniques noted above, the transfer of droplets iscaused to occur by different modes or patterns for the pulse arc weldingand the short-circuit transfer arc welding. Accordingly, upon switchingbetween the pulse arc welding and the short-circuit transfer arcwelding, spatters may be generated and the welding state may becomeunstable.

SUMMARY

In view of the above circumstances, the present disclosure aims toprovide an arc welding method that facilitates smooth switching betweenpulse arc welding and short-circuit transfer arc welding.

According to an aspect of the present disclosure, there is provided anarc welding method that includes: performing pulse arc welding with awelding wire being fed in a forward direction during a first period;performing short-circuit transfer arc welding with the welding wirebeing fed in the forward direction and a reverse direction during asecond period; and alternately switching between the first period andthe second period. The switching of the first period to the secondperiod is performed during a peak period of the pulse arc welding.

Preferably, the method further includes increasing a feeding speed ofthe welding wire to a forward-feeding peak value during the peak periodof the pulse arc welding.

Preferably, the forward-feeding peak value in a first cycle of thesecond period is set to be different from the forward-feeding peak valuein a subsequent cycle of the second period.

Preferably, the short-circuit transfer arc welding is switched to thepulse arc welding during an arc period of the short-circuit transfer arcwelding.

Preferably, the short-circuit transfer arc welding is switched to thepulse arc welding at a timing when arching reoccurs and a weldingcurrent is at a relatively low level during the short-circuit transferarc welding.

Further features and advantages of the present disclosure will becomeapparent from the following detailed description with reference to theattached drawings.

DRAWINGS

FIG. 1 is a block diagram showing a welding power supply forimplementing an arc welding method according to an embodiment of thepresent disclosure;

FIG. 2 is a timing chart showing signals for switching from a pulse arcwelding period Ta to a short-circuit transfer arc welding period Tc inthe welding power supply of FIG. 1; and

FIG. 3 is a timing chart showing signals for switching from ashort-circuit transfer arc welding period Tc to a pulse arc weldingperiod Ta in the welding power supply of FIG. 1.

EMBODIMENTS

The following describes embodiments of the present disclosure withreference to the accompanying drawings.

FIG. 1 is a block diagram showing a welding power supply forimplementing an arc welding method according to an embodiment of thepresent disclosure. The following describes the blocks illustrated inFIG. 1.

A power main circuit PM may receive a power of three-phase 200 V (or anyother suitable voltages) from a commercial power source, and performsoutput control such as inverter control based on an error amplificationsignal Ea (described later) so as to output a voltage E. Typically, butwithout limitation, the power main circuit PM may include a primaryrectifier, a smoothing capacitor, an inverter circuit, a high-frequencytransformer, and a secondary rectifier. The primary rectifier rectifiesalternating current from the commercial power source and outputs therectified current or direct current (DC). The smoothing capacitorsmoothes the direct current from the primary rectifier. The invertercircuit is driven by the error amplification signal Ea and converts thesmoothed DC into high-frequency AC. The high-frequency transformerlowers the voltage of the high-frequency AC to a value suitable forwelding. The secondary rectifier rectifies the lowered high-frequency ACto DC.

A reactor WL smoothes (e.g., removes undesired ripples from) a weldingcurrent Iw so that an arc 3 will be maintained in a stable state.

A feeding motor WM receives a feeding control signal Fc (describedlater), and feeds a welding wire 1 at feeding speed Fw. The feedingmotor WM performs forward feeding during the pulse arc welding periodand performs forward and reverse feeding during the short-circuittransfer arc welding period. The feeding motor WM has high transientresponsiveness. The feeding motor WM may be provided near the tip of awelding torch 4, in order to improve the rate of change in the feedingspeed Fw of the welding wire 1 and to enable quick inversion of the wirefeeding direction. In an embodiment, two feeding motors WM may be usedto form a push-pull feeding system.

The welding wire 1 is fed through the welding torch 4 by the rotation ofa feeding roll 5 connected to the feeding motor WM, and the arc 3 isgenerated between the tip of the welding wire 1 and a base material 2.Welding voltage Vw is applied between a power supply chip (not shown)within the welding torch 4 and the base material 2 to cause the weldingcurrent Iw to flow. A shield gas (not shown) is ejected from the tip ofthe welding torch 4 to shield the arc 3 from the atmosphere. For theshield gas, a mixed gas of argon gas and carbon dioxide gas may be usedwhen the welding wire 1 is made of steel, while argon gas may be usedwhen the welding wire 1 is made of aluminum.

An output voltage setting circuit ER outputs a predetermined outputvoltage setting signal Er. An output voltage detection circuit EDdetects the output voltage E and smoothes it to output an output voltagedetection signal Ed.

A voltage error amplification circuit EV receives the output voltagesetting signal Er and the output voltage detection signal Ed, amplifiesthe error between the output voltage setting signal Er (+) and theoutput voltage detection signal Ed (−), and outputs a voltage erroramplification signal Ev.

A current detection circuit ID detects the welding current Iw andoutputs a current detection signal Id. A voltage detection circuit VDdetects the welding voltage Vw and outputs a voltage detection signalVd. A short-circuit determination circuit SD receives the voltagedetection signal Vd, and outputs a short-circuit determination signalSd. Specifically, when the value of the voltage detection signal Vd isless than a predetermined short-circuit determination value(approximately 10 V), the short-circuit determination circuit SDdetermines that the current period is a short-circuit period, andoutputs a short-circuit determination signal Sd of High level. When thevalue of the voltage detection signal Vd is greater than or equal to thepredetermined short-circuit determination value, the short-circuitdetermination circuit SD determines that the current period is an arcperiod, and outputs a short-circuit determination signal Sd of Lowlevel.

A forward-feeding acceleration period setting circuit TSUR outputs apredetermined forward-feeding acceleration period setting signal Tsur.

A forward-feeding deceleration period setting circuit TSDR outputs apredetermined forward-feeding deceleration period setting signal Tsdr.

A reverse-feeding acceleration period setting circuit TRUR outputs apredetermined reverse-feeding acceleration period setting signal Trur.

A reverse-feeding deceleration period setting circuit TRDR outputs apredetermined reverse-feeding deceleration period setting signal Trdr.

A forward-feeding peak value setting circuit WSR receives a timer signalTm (described later) and the short-circuit determination signal Sd, andoutputs a forward-feeding peak value setting signal Wsr. Theforward-feeding peak value setting signal Wsr may indicate twopredetermined values, for example, depending on the situations.Specifically, the forward-feeding peak value setting signal Wsr mayindicate: an initial value during a period (“initial value period”) fromwhen the timer signal Tm changes to Low level (short-circuit transferarc welding period Tc) to when the short-circuit determination signal Sdfirst changes to High level (short-circuit period); and a steady-statevalue during periods other than the initial value period.

A reverse-feeding peak value setting circuit WRR outputs a predeterminedreverse-feeding peak value setting signal Wrr.

A short-circuit arc feeding speed setting circuit FCR receives signalssuch as the forward-feeding acceleration period setting signal Tsur, theforward-feeding deceleration period setting signal Tsdr, thereverse-feeding acceleration period setting signal Trur, thereverse-feeding deceleration period setting signal Trdr, theforward-feeding peak value setting signal Wsr, the reverse-feeding peakvalue setting signal Wrr, and the short-circuit determination signal Sd.Based on these signals, the short-circuit arc feeding speed settingcircuit FCR outputs a short-circuit arc feeding speed setting signal Fcrhaving the following feeding speed pattern (1-7). As seen from thebelow, the short-circuit arc feeding speed setting signal Fcr having apositive value corresponds to a forward-feeding period, whereas theshort-circuit arc feeding speed setting signal Fcr having a negativevalue corresponds to a reverse-feeding period.

(1) During a forward-feeding acceleration period Tsu (determined by theforward-feeding acceleration period setting signal Tsur), theshort-circuit arc feeding speed setting signal Fcr linearly increases(accelerates) from zero (or from the value of a pulse feeding speedsetting signal Far if it is immediately after the period switching to ashort-circuit transfer arc welding period Tc has occurred) to aforward-feeding peak value Wsp that is a positive value determined bythe forward-feeding peak value setting signal Wsr. Accordingly, as shownin FIG. 2 (A), switching the first period (i.e. the pulse arc weldingperiod Ta) to the second period (i.e. the short-circuit transfer arcwelding period Tc) is performed prior to a peak period Wsp of theshort-circuit transfer arc welding and also when a welding voltage and awelding current have peak values.

(2) During a forward-feeding peak period Tsp, the short-circuit arcfeeding speed setting signal Fcr maintains the forward-feeding peakvalue Wsp.

(3) During a forward-feeding deceleration period Tsd (determined by theforward-feeding deceleration period setting signal Tsdr), theshort-circuit arc feeding speed setting signal Fcr linearly decreases(decelerates) from the forward-feeding peak value Wsp to zero. As seenfrom FIG. 2, the forward-feeding deceleration period Tsd starts when theshort-circuit determination signal Sd changes from Low level (arcperiod) to High level (short-circuit period).

(4) During a reverse-feeding acceleration period Tru (determined by thereverse-feeding acceleration period setting signal Trur), theshort-circuit arc feeding speed setting signal Fcr linearly acceleratesfrom zero to a reverse-feeding peak value Wrp that is a negative valuedetermined by the reverse-feeding peak value setting signal Wrr.

(5) During a reverse-feeding peak period Trp, the short-circuit arcfeeding speed setting signal Fcr maintains the reverse-feeding peakvalue Wrp. Accordingly, as shown in FIG. 2 (A), the second period (i.e.the short-circuit transfer arc welding period Tc) has at least twodifferent periods (Tp/Tsp; Tp/Trp; Tp/Tsp) during which the welding wireis fed at a constant feeding speed. The at least two different periodsinclude a forward-feeding period (Tp/Tsp) and a reverse-feeding period(Tp/Trp), and the constant feeding speed Wsp of the forward-feedingperiod (Tp/Tsp) is greater than a feeding speed of the welding wire atthe time of switching from the first period to the second period.

(6) During a reverse-feeding deceleration period Trd (determined by thereverse-feeding deceleration period setting signal Trdr), theshort-circuit arc feeding speed setting signal Fcr linearly deceleratesfrom the reverse-feeding peak value Wrp to zero. As seen from FIG. 2,the reverse-feeding deceleration period Trd stars when the short-circuitdetermination signal Sd changes from High level (short-circuit period)to Low level (arc period).

(7) The set of the above steps (1) to (6) may be repeated a suitablenumber of times. Thus, the short-circuit arc feeding speed settingsignal Fcr will have a pattern (“feeding pattern”) that changes in theform of positive and negative trapezoidal waves.

A current limiting resistor R is provided between the reactor WL and thewelding torch 4. The current limiting resistor R may have a value(approximately 0.5 to 3Ω) at least 50 times larger than the resistancevalue (approximately 0.01 to 0.03Ω) of the conduction path of thewelding current Iw during the short-circuit period. When the currentlimiting resistor R is inserted in the current path of the weldingcurrent Iw (as described later), the energy accumulated in the reactorWL and the reactor of the welding cable is consumed rapidly.

A transistor TR is connected in parallel to the current limitingresistor R, and is subjected to ON/OFF control according to a drivesignal Dr (described later).

A constriction detection circuit ND receives the short-circuitdetermination signal Sd, the voltage detection signal Vd, and thecurrent detection signal Id, and outputs a constriction detection signalNd. When the short-circuit determination signal Sd is at High level(short-circuit period) and the voltage value of the voltage detectionsignal Vd reaches a reference value, the constriction detection circuitND determines that the forming state of a constriction in the weldingwire has reached a predetermined reference state, and outputs theconstriction detection signal Nd at High level. When the short-circuitdetermination signal Sd changes to Low level (arc period), theconstriction detection circuit ND outputs the constriction detectionsignal Nd at Low level. As another example, when the differential valueof the voltage detection signal Vd during the short-circuit periodreaches the corresponding reference value, the constriction detectionsignal Nd may be changed to High level. As yet another example, theresistance value of a droplet may be calculated by dividing the value ofthe voltage detection signal Vd by the value of the current detectionsignal Id, and when the differential value of the resistance valuereaches the corresponding reference value, the constriction detectionsignal Nd may be changed to High level.

A low-level current setting circuit ILR outputs a predeterminedlow-level current setting signal Ilr. A current comparison circuit CMreceives the low-level current setting signal Ilr and the currentdetection signal Id, and outputs a current comparison signal Cm. Thecurrent comparison signal Cm is at High level when Id<Ilr, and is at Lowlevel when Id≥Ilr.

A drive circuit DR receives the current comparison signal Cm and theconstriction detection signal Nd, and outputs the drive signal Dr to thebase of the transistor TR. The drive signal Dr changes to Low level whenthe constriction detection signal Nd changes to High level, and thenchanges to High level when the current comparison signal Cm changes toHigh level. In this manner, the drive signal Dr becomes Low level upondetection of a constriction, thereby causing the transistor TR to beturned off (or placed in OFF state), and thus the current limitingresistor R is inserted in the current path. As a result, the weldingcurrent Iw rapidly decreases. When the welding current Iw is reduced to(or lower than) the value of the low-level current setting signal Ilr,the drive signal Dr changes to High level and the transistor TR isturned on (placed in ON state). As a result, the current limitingresistor R is short-circuited, and the normal state is restored.

A short-circuit arc current setting circuit ICR receives theshort-circuit determination signal Sd, the low-level current settingsignal Ilr, and the constriction detection signal Nd, and performs thefollowing processing to output the following short-circuit arc currentsetting signal Icr.

(1) When the short-circuit determination signal Sd is at Low level (arcperiod), the short-circuit arc current setting signal Icr is thelow-level current setting signal Ilr. In other words, the low-levelcurrent setting signal Ilr is outputted from the circuit ICR as theshort-circuit arc current setting signal Icr.

(2) When the short-circuit determination signal Sd changes to High level(short-circuit period), the short-circuit arc current setting signal Icrto be outputted indicates a predetermined initial current setting valueduring a predetermined initial period, and then (after the initialperiod) increases from the initial current setting value to apredetermined short-circuit peak setting value with a predeterminedshort-circuit inclination. The short-circuit arc current setting signalIcr maintains the short-circuit peak setting value for a while.

(3) Thereafter, when the constriction detection signal Nd changes toHigh level, the short-circuit arc current setting signal Icr indicatesthe value of the low-level current setting signal Ilr.

A current drop time setting circuit TDR outputs a current drop timesetting signal Tdr.

A small-current period circuit STD receives the short-circuitdetermination signal Sd and the current drop time setting signal Tdr,and outputs a small-current period signal Std. The small-current periodsignal Std becomes High level when the time determined by the currentdrop time setting signal Tdr has elapsed from when the short-circuitdetermination signal Sd changes to Low level (arc period). Thereafter,when the short-circuit determination signal Sd changes to High level(short-circuit period), the small-current period signal Std changes toLow level.

A pulse cycle circuit TF receives the voltage error amplification signalEv, performs voltage/frequency conversion on the voltage erroramplification signal Ev, and outputs a pulse cycle signal Tf thatbecomes High level for a short period of time for every pulse cycle. Thepulse cycle signal Tf determines the repetition cycle of the peak periodand the base period of the pulse arc welding.

A peak current setting circuit IPR outputs a predetermined peak currentsetting signal Ipr. A base current setting circuit IBR outputs apredetermined base current setting signal Ibr.

A pulse current setting circuit IAR receives the pulse cycle signal Tf,the peak current setting signal Ipr, and the base current setting signalIbr, and performs the following processing to output the following pulsecurrent setting signal Iar.

(1) When the pulse cycle signal Tf changes to High level for a shortperiod of time, the pulse current setting signal Iar rises from the basecurrent setting signal Ibr to the peak current setting signal Ipr duringa predetermined peak rise period Tu.

(2) During a predetermined peak period Tp, the pulse current settingsignal Iar is the peak current setting signal Ipr.

(3) During a predetermined peak fall period Td, the pulse currentsetting signal Iar falls from the peak current setting signal Ipr to thebase current setting signal Ibr.

(4) During the base period Tb that continues until the pulse cyclesignal Tf becomes High level (for a short period of time), the pulsecurrent setting signal Iar is the base current setting signal Ibr.

A pulse arc welding period setting circuit TAR outputs a predeterminedpulse arc welding period setting signal Tar. A short-circuit transferarc welding period setting circuit TCR outputs a predeterminedshort-circuit transfer arc welding period setting signal Tcr.

A timer circuit TM receives the pulse arc welding period setting signalTar, the short-circuit transfer arc welding period setting signal Tcr,the short-circuit determination signal Sd, the pulse current settingsignal Iar, and the peak current setting signal Ipr. Based on thesesignals, the time circuit TM outputs a timer signal Tm.

As shown in FIG. 2 (F), the timer signal Tm changes to Low level (seet1) and maintains the Low level (short-circuit transfer arc weldingperiod Tc). Then, a certain time period determined by the short-circuittransfer arc welding period setting signal Tcr will elapse andthereafter the short-circuit determination signal Sd will change to Lowlevel (see signal Sd shown in FIG. 3 (D), in which the signal Sd changesto Low level (arc period) for the first time after the elapse of thetime period determined by the signal Tcr). Further, a predetermineddelay period elapses after the change of the short-circuit determinationsignal Sd, and at this point, the timer signal Tm changes to High level(see t1 in FIG. 3) and maintains the High level (pulse arc weldingperiod Ta).

After the timer signal Tm changes to High level (t1 in FIG. 3), a perioddetermined by a pulse arc welding period setting signal Tar will elapse,and the pulse current setting signal Iar becomes equal in value to thepeak current setting signal Ipr (i.e., these two signals become equalfor the first time after the lapse of the period determined by thesetting signal Tar). At this point, the timer signal Tm changes to Lowlevel (see t1 in FIG. 2).

In light of the above, the pulse arc welding period Ta is the sum of theperiod by the pulse arc welding period setting signal Tar, and thesubsequent period which continues until the first peak period begins.The short-circuit transfer arc welding period Tc is the sum of theperiod by the short-circuit transfer arc welding period setting signalTcr and the subsequent period which continues until the first delayperiod ends.

A pulse feeding speed setting circuit FAR outputs a pulse feeding speedsetting signal Far having a predetermined positive value.

A feeding speed setting circuit FR receives the timer signal Tm, theshort-circuit arc feeding speed setting signal Fcr, and the pulsefeeding speed setting signal Far. Then, the feeding speed settingcircuit FR outputs the pulse feeding speed setting signal Far as afeeding speed setting signal Fr when the timer signal Tm is at Highlevel (pulse arc welding period Ta), and outputs the short-circuit arcfeeding speed setting signal Fcr as the feeding speed setting signal Frwhen the timer signal Tm is at Low level (short-circuit transfer arcwelding period Tc).

A feeding control circuit FC receives the feeding speed setting signalFr, and outputs, to the feeding motor WM, the feeding control signal Fcfor feeding the welding wire 1 at the feeding speed Fw corresponding tothe value indicated by the feeding speed setting signal Fr.

A current setting circuit IR receives the timer signal Tm, theshort-circuit arc current setting signal Icr, and the pulse currentsetting signal Iar. The current setting circuit IR outputs the pulsecurrent setting signal Iar as a current setting signal Ir when the timersignal Tm is at High Level (pulse arc welding period Ta), and outputsthe short-circuit arc current setting signal Icr as the current settingsignal Ir when the timer signal Tm is at Low level (short-circuittransfer arc welding period Tc).

A current error amplification circuit EI receives the current settingsignal Ir and the current detection signal Id, amplifies the errorbetween the current setting signal Ir (+) and the current detectionsignal Id (−), and outputs a current error amplification signal Ei.

A power characteristic switching circuit SW receives the timer signalTm, the current error amplification signal Ei, the voltage erroramplification signal Ev, the short-circuit determination signal Sd, andthe small-current period signal Std. Based on these signals, the powercharacteristic switching circuit SW performs the following processing tooutput the following error amplification signal Ea.

(1) During the period from when the short-circuit determination signalSd changes to High level (short-circuit period) while the timer signalTm is being at Low level to when the delay period has lapsed after theshort-circuit determination signal Sd changes to Low level (arc period),the current error amplification signal Ei is outputted as the erroramplification signal Ea.

(2) During a subsequent large-current arc period, the voltage erroramplification signal Ev is outputted as the error amplification signalEa.

(3) During a small-current arc period in which the small-current periodsignal Std becomes High level during the arc period, the current erroramplification signal Ei is outputted as the error amplification signalEa.

(4) During the period in which the timer signal Tm is at High level, thecurrent error amplification signal Ei is outputted as the erroramplification signal Ea.

With the above arrangements, the welding power supply in theshort-circuit transfer arc welding period Tc has a constant currentcharacteristic during the short-circuit period, the delay period, andthe small-current arc period. On the other hand, the welding powersupply has a constant voltage characteristic during the other period,i.e., the large-current arc period (a period while the timer signal Tmis being at Low level, from when the delay time has elapsed after theshort-circuit determination signal Sd changes from High level to Lowlevel to when the small-current period signal Std changes from Low levelto High level). During the pulse arc welding period Ta, the weldingpower supply has a constant current characteristic.

FIG. 2 is a timing chart showing several signals for switching from thepulse arc welding period Ta to the short-circuit transfer arc weldingperiod Tc in the welding power supply of FIG. 1. In FIG. 2, (A) showsthe change of the feeding speed Fw with time, (B) shows the change ofthe welding current Iw with time, (C) shows the change of the weldingvoltage Vw with time, (D) shows the change of the short-circuitdetermination signal Sd with time, (E) shows the change of thesmall-current period signal Std with time, and (F) shows the change ofthe timer signal Tm with time. With reference to FIG. 2 (and FIG. 3 aswell), the following describes the operations by the respective signals.

As shown in (B) of FIG. 2, the welding current Iw begins to rise at timet0 from the base current and increases up to the peak current value attime t1. At the same time (i.e., at t1), the timer signal Tm changesfrom High level to Low level. Specifically, before time t1 (and t0, forthat matter), the timer signal Tm may change to High level (see t1 inFIG. 3, at which the pulse arc welding period Ta starts), and then thetime determined by the pulse arc welding period setting signal Tarelapses. Thereafter, the pulse current setting signal Iar becomes equalto the peak current setting signal Ipr for the first time. At thistiming, as shown in (F) of FIG. 2, the timer signal Tm changes from Highlevel to Low level (see time t1 in FIG. 2). Accordingly, at time t1(FIG. 2), the pulse arc welding period Ta is changed to theshort-circuit transfer arc welding period Tc. As shown in (A) of FIG. 2,forward feeding is performed during the period before time t1 with thefeeding speed Fw being a constant speed determined by the pulse feedingspeed setting signal Far shown in FIG. 1. As shown in (C) of FIG. 2, thewelding voltage Vw increases from the base voltage up to the peakvoltage. As shown in (D) of FIG. 2, the short-circuit determinationsignal Sd remains at Low level because the arc period continues. Asshown in (E) of FIG. 2, the small-current period signal Std remains atLow level.

As shown in (F) of FIG. 2, the timer signal Tm changes to Low level attime t1 and the short-circuit transfer arc welding period Tc begins. Inresponse, as shown in (A) of FIG. 2, the feeding speed Fw increases tothe forward-feeding peak value Wsp determined by the forward-feedingpeak value setting signal Wsr shown in FIG. 1, and maintains the valueuntil short-circuiting occurs at time t3. The forward-feeding peak valueWsp during this period is a predetermined “initial value” because it iswithin a particular period, i.e., from when the timer signal Tm changesto Low level to when the short-circuit determination signal Sd changesto High level (short-circuit period) for the first time immediatelyafter the changing of the timer signal Tm to Low level. Thereafter, theforward-feeding peak value Wsp may be a predetermined “steady-state”value. Preferably, the initial value is determined independently fromthe steady-state value so as to stabilize the welding state during thisparticular period. In an embodiment, however, the initial value may beequal to the steady-state value.

During the period from t1 to t2 (FIG. 2), the welding current Iw willchange in response to the variation of the arc load since the weldingpower source is in the state of the constant voltage characteristic. Forinstance, the welding current Iw may begin to decrease at time t1.Likewise, as shown in (C) of FIG. 2, the welding voltage Vw may alsobegin to fall.

At time t2 (FIG. 2), the time of lapse from t1 reaches the valuedetermined by the current drop time setting signal Tdr. Thus, as shownin (E) of FIG. 2, the small-current period signal Std changes from Lowlevel to High level. In response, the mode of the welding power sourceis switched from the constant voltage characteristic to the constantcurrent characteristic. Hence, as shown in (B) of FIG. 2, the weldingcurrent Iw decreases to the low level current value, and maintains thatlevel until short-circuiting occurs at time t3. Likewise, as shown in(C) of FIG. 2, the welding voltage Vw decreases. The small-currentperiod signal Std changes to Low level from High level at time t3, i.e.,when short-circuiting occurs.

The feeding speed Fw shown in (A) of FIG. 2 is controlled by theshort-circuit arc feeding speed setting signal Fcr outputted from theshort-circuit arc feeding speed setting circuit FCR shown in FIG. 1. Thefeeding speed Fw can be divided into several sections that correspondto: the forward-feeding acceleration period Tsu determined by theforward-feeding acceleration period setting signal Tsur; theforward-feeding peak period Tsp that continues until short-circuitingoccurs; the forward-feeding deceleration period Tsd determined by theforward-feeding deceleration period setting signal Tsdr; thereverse-feeding acceleration period Tru determined by thereverse-feeding acceleration period setting signal Trur; thereverse-feeding peak period Trp that continues until arcing occurs; andthe reverse-feeding deceleration period Trd determined by thereverse-feeding deceleration period setting signal Trdr. Theforward-feeding peak value Wsp is determined by the forward-feeding peakvalue setting signal Wsr, and the reverse-feeding peak value Wrp isdetermined by the reverse-feeding peak value setting signal Wrr. Theshort-circuit arc feeding speed setting signal Fcr has a feeding patternthat changes in the form of positive and negative trapezoidal waves.

Operations in Short-Circuit Period from t3 to t6

When short-circuiting occurs at time t3 in the forward-feeding peakperiod Tsp, the welding voltage Vw rapidly decreases to a short-circuitvoltage value of several volts, as shown in (C) of FIG. 2. This causesthe short-circuit determination signal Sd to change to High level(short-circuit period), as shown in (D) of FIG. 2. In response, thefeeding speed Fw enters the forward-feeding deceleration period Tsd fromtime t3 to t4, and decelerates from the forward-feeding peak value Wspto zero, as shown (A) of FIG. 2. The forward-feeding deceleration periodTsd is set to 1 ms (Tsd=1 ms), for example.

As shown in (A) of FIG. 2, the feeding speed Fw enters thereverse-feeding acceleration period Tru from time t4 to time t5, andaccelerates from zero to the reverse-feeding peak value Wrp. Theshort-circuit period continues during this period Tru. Thereverse-feeding acceleration period Tru is set to 1 ms (Tru=1 ms), forexample.

When the reverse-feeding acceleration period Tru ends at time t5, thefeeding speed Fw enters the reverse-feeding peak period Trp andindicates the reverse-feeding peak value Wrp, as shown in (A) of FIG. 2.The reverse-feeding peak period Trp continues until arcing occurs attime t6. Accordingly, the period from time t3 to time t6 is ashort-circuit period. The reverse-feeding peak period Trp may not bespecifically set, but may last approximately 4 ms. The reverse-feedingpeak value Wrp may be set to −60 m/min (Wrp=−60 m/min), for example.

Referring to (B) of FIG. 2, the welding current Iw may have apredetermined initial current value during a predetermined initialperiod (or sub-period) in the short-circuit period from time t3 to t6.Then, the welding current Iw may rise with a predetermined short-circuitinclination (in other words, within a short-circuit inclination period)to reach a predetermined short-circuit peak value, and may maintain (orsubstantially maintain) the short-circuit peak value.

As shown in (C) of FIG. 2, the welding voltage Vw will begin to rise ator around the point when the welding current Iw reaches theshort-circuit peak value. This is because a constriction is graduallyformed in a droplet at the tip of the welding wire 1 due to the reversefeeding of the welding wire 1 and the action of a pinch force by thewelding current Iw.

Then, when the voltage value of the welding voltage Vw reaches thereference value, determination is made that the constriction beingformed has reached the reference state, and accordingly the constrictiondetection signal Nd of FIG. 1 changes to High level.

In response to the state where the constriction detection signal Ndindicates High level, the drive signal Dr of FIG. 1 indicates Low level.This causes the transistor TR of FIG. 1 to be turned off, and thecurrent limiting resistor R of FIG. 1 is inserted in the current path.At the same time, the value of the short-circuit arc current settingsignal Icr decreases to the value of the low-level current settingsignal Ilr. As a result, the value of the welding current Iw decreasessharply or plummets from the short-circuit peak value to a low-levelcurrent value, as shown in (B) of FIG. 2. When the welding current Iwdecreases to the low-level current value, the drive signal Dr changesback to High level, thereby causing the transistor TR to be turned on,and hence the current limiting resistor R to be short-circuited. Asshown in (B) of FIG. 2, the welding current Iw maintains the low-levelcurrent value until a predetermined delay period elapses from thereoccurrence of arcing, for the short-circuit arc current setting signalIcr is still the low-level current setting signal Ilr. Thus, thetransistor TR is placed in OFF state only during the period from whenthe constriction detection signal Nd changes to High level to when thewelding current Iw decreases to the low-level current value. As shown in(C) of FIG. 2, the welding voltage Vw decreases once and then rapidlyrises due to the decrease in the welding current Iw. The parametersdescribed above may be set to have the following values. Initialcurrent=40 A, Initial period=0.5 ms, Short-circuit inclination=180 A/ms,Short-circuit peak value=400 A, Low-level current value=50 A, and Delayperiod=0.5 ms.

Operations in Arc Period from t6 to t9

The reverse feeding of the welding wire and the action of the pinchforce by the flow of the welding current Iw cause the constriction toprogress and generate an arc at time t6, and accordingly the weldingvoltage Vw rapidly increases to an arc voltage value of several tens ofvolts, as shown in (C) of FIG. 2. As a result, the short-circuitdetermination signal Sd changes to Low level (arc period), as shown in(D) of FIG. 2. In response, the feeding speed Fw enters thereverse-feeding deceleration period Trd from time t6 to time t7, anddecelerates from the reverse-feeding peak value Wrp to zero, as shown in(A) of FIG. 2.

When the reverse-feeding deceleration period Trd ends at time t7, thefeeding speed Fw enters the forward-feeding acceleration period Tsu fromtime t7 to time t8. During the forward-feeding acceleration period Tsu,the feeding speed Fw accelerates from zero to the forward-feeding peakvalue Wsp, as shown in (A) of FIG. 2. The arc period continues duringthis period. The forward-feeding acceleration period Tsu is set to 1 ms(Tsu=1 ms), for example.

When the forward-feeding acceleration period Tsu ends at time t8, thefeeding speed Fw enters the forward-feeding peak period Tsp, andindicates the forward-feeding peak value Wsp, as shown in (A) in FIG. 2.The arc period still continues during this period. The forward-feedingpeak period Tsp continues until short-circuiting occurs at time t9.Accordingly, the arc period is the period from time t6 to time t9. Then,when short-circuiting occurs, the same operations as those at time t3onward will be repeated. The forward-feeding peak period Tsp may not bespecifically set, but may last approximately 4 ms. The forward-feedingpeak value Wsp may be set to 70 m/min (Wsp=70 m/min), for example.

When arcing occurs at time t6, the welding voltage Vw rapidly increasesto an arc voltage value of several tens of volts, as shown in (C) ofFIG. 2. On the other hand, the welding current Iw maintains thelow-level current value during the delay period from time t6 to timet61. After time t61, the welding current Iw rapidly increases to thepeak value, and then reaches a large current value that graduallydecreases. During the large-current arc period from time t61 to t81, thevoltage error amplification signal Ev of FIG. 1 performs feedbackcontrol for the welding power supply, so that the welding power supplyhas a constant voltage characteristic. Accordingly, the value of thewelding current Iw during the large-current arc period changes with thearc load.

As shown in (E) of FIG. 2, the small-current period signal Std changesto High level at time t81 when the current drop time determined by thecurrent drop time setting signal Tdr of FIG. 1 elapses from theoccurrence of arcing at time t6. In response, the welding power supplyis switched to have a constant current characteristic instead of theconstant voltage characteristic. As a result, the welding current Iwdecreases to the low-level current value, and maintains the value untiltime t9 at which short-circuiting occurs. Similarly, the welding voltageVw also decreases as shown in (C) of FIG. 2. The small-current periodsignal Std changes back to Low level when short-circuiting occurs attime t9.

The short-circuit transfer arc welding period Tc includes a plurality ofcycles in which a short-circuit period and an arc period are alternatelyrepeated. One cycle of short-circuiting and arcing lasts approximately10 ms, for example. The short-circuit transfer arc welding period Tclasts approximately 50 to 500 ms, for example. The example shown in FIG.2 illustrates a case where the switching to the short-circuit transferarc welding period Tc is performed at the start of a peak period.Alternatively, the switching may be performed sometime in the middle ofthe peak period Tp, or during the peak rise period Tu, or the peak fallperiod Td.

FIG. 3 is a timing chart showing several signals for switching from theshort-circuit transfer arc welding period Tc to the pulse arc weldingperiod Ta in the welding power supply of FIG. 1. In FIG. 3, (A) showsthe change of the feeding speed Fw with time, (B) shows the change ofthe welding current Iw with time, (C) shows the change of the weldingvoltage Vw with time, (D) shows the change of the short-circuitdetermination signal Sd with time, (E) shows the change of thesmall-current period signal Std with time, and (F) shows the change ofthe timer signal Tm with time. With reference to FIG. 3, the followingdescribes the operations by the respective signals.

At time t1 (and immediately before time t1), the welding current Iw hasa low-level current value, as shown in (B) of FIG. 3. This is becausethe welding current Iw is in a delay period that comes after the end ofshort-circuiting and reoccurrence of arcing. As shown in (F) of FIG. 3,the timer signal Tm changes from Low level to High level at time t1since this is the timing at which the predetermined delay period haselapsed, starting from when the short-circuit determination signal Sdchanges to Low level (see (D) of FIG. 3) for the first time after theperiod determined by the short-circuit transfer arc welding periodsetting signal Tcr (FIG. 1) has elapsed (this period starts from whenthe timer signal Tm changes to Low level. See t1 in FIG. 2). As aresult, the short-circuit transfer arc welding period Tc is switched tothe pulse arc welding period Ta at t1 in FIG. 3. As shown in (A) of FIG.3, before time t1, the feeding speed Fw is in the reverse-feedingdeceleration period Trd in which the wire feeding speed decreases fromthe reverse-feeding peak value Wrp toward zero. As shown in (C) of FIG.3, the welding voltage Vw has an arc voltage value. As shown in (D) ofFIG. 3, the short-circuit determination signal Sd is at Low level duringthe delay period. As shown in (E) of FIG. 3, the small-current periodsignal Std remains at Low level.

As shown in (F) of FIG. 3, the timer signal Tm changes to High level attime t1 and the pulse arc welding period Ta begins. In response, asshown in (A) of FIG. 3, forward feeding is performed with the feedingspeed Fw being a constant speed determined by the pulse feeding speedsetting signal Far shown in FIG. 1.

As shown in (B), during the predetermined peak rise period Tu from timet1 to time t2, a transition current rising to a predetermined peakcurrent value Ip flows. During the predetermined peak period Tp fromtime t2 to time t3, current of the peak current value Ip flows. Duringthe predetermined peak fall period Td from time t3 to time 4, atransition current falling from the peak current value Ip to thepredetermined base current value Ib flows. During the base period Tbfrom time t4 to time t5, the current having the base current value Ibflows. During the pulse arc welding period Ta, the welding power supplyhas a constant current characteristic, and hence the welding current Iwis set by the pulse current setting signal Iar shown in FIG. 1. As shownin (C) of FIG. 3, the welding voltage Vw has a waveform similar to thecurrent waveform. The pulse cycle Tf from time t1 to time t5 issubjected to feedback control so that the welding voltage Vw has adesired average value. The current waveform parameter is set such thatone droplet is transferred for each pulse cycle Tf. For example, thepeak current Ip is set to 500 A, the base current Ib to 60 A, the peakrise period Tu to 1 ms, the peak period Tp to 2 ms, and the peak fallperiod Td to 1 ms.

The pulse arc welding period Ta includes a plurality of pulse cycles Tf.Each pulse cycle Tf lasts approximately 15 ms, for example. The pulsearc welding period Ta lasts approximately 50 to 500 ms, for example.FIG. 3 shows the case where switching to the pulse arc welding period Tais performed during the delay period. Alternatively, the pulse arcwelding period Ta may begin from the base period Tb. Preferably, asmall-current period may be provided before the first peak current Ipflows.

The following explains the advantages of the present embodiment.According to the present embodiment, switching to the short-circuittransfer arc welding period is performed during a peak period in thepulse arc welding. In other words, the switching to the short-circuittransfer arc welding period is performed during the arc period in whicha large current flows. Accordingly, a droplet is formed on the weldingwire before time t3, i.e., before the first short-circuiting occurs inthe short-circuit transfer arc welding period. As a result, the dropletcan be smoothly transferred to the molten pool during theshort-circuiting period, whereby switching from the pulse arc weldingperiod to the short-circuit transfer arc welding period is properlyperformed. If otherwise, i.e., short-circuiting occurs with no dropletformed, the non-melted portion of the welding wire will thrust into themolten pool, which makes the welding state unstable and generates largespatters. According to the present embodiment, it is possible to preventsuch undesired conditions from occurring.

Furthermore, according to the present embodiment, the feeding speedbegins to increase during the peak period of the pulse arc weldingtoward the peak value, while the switching to the short-circuit transferarc welding period is performed. In this way, fast forward-feeding canbe performed with the forward-feeding peak value, thus preventing theoccurrence of the first short-circuiting from taking an unduly longtime. In addition, it is possible to prevent the size of the dropletformed on the wire from becoming too large at the timing when the firstshort-circuiting occurs, which is advantageous to suppressing theoccurrence of spatters.

Furthermore, according to the present embodiment, the forward-feedingpeak value in the first cycle during the short-circuit transfer arcwelding may be different from the forward-feeding peak value in thesubsequent cycles. In this way, the forward-feeding peak value beforethe occurrence of the first short-circuiting can be set to anappropriate value, which is advantageous to optimizing the size of thedroplet at the timing when the first short-circuiting occurs, andtherefore to preventing more effectively the occurrence of spatters.

Furthermore, as seen from the situations at time t1 of FIG. 3, switchingto the pulse arc welding is performed during the arc period of theshort-circuit transfer arc welding. If such switching is performedduring the short-circuit period, it will render the droplet transferunstable, and my lead to an unstable welding state and occurrence oflarge drops of spatter. Switching during the arc period, on the otherhand, allows smooth transition to the pulse arc welding.

Furthermore, according to the present embodiment, the switching to thepulse arc welding is performed in a state of low level current valuewith arcing reoccurring during the short-circuit transfer arc welding.In this way, switching to the pulse arc welding is performed with nodroplet being formed, which makes it possible to realize a one pulsecycle-one droplet transfer state from the first pulse cycle. This canfurther stabilize the welding state.

The invention claimed is:
 1. An arc welding method comprising:performing pulse arc welding with a welding wire being fed in a forwarddirection during a first period; performing short-circuit transfer arcwelding with the welding wire being fed in the forward direction and areverse direction during a second period; and alternately switchingbetween the first period and the second period, wherein switching thefirst period to the second period is performed prior to a peak period ofthe short-circuit transfer arc welding and when a welding voltage and awelding current reach peak values, and the second period has at leasttwo different periods during which the welding wire is fed at a constantfeeding speed; the at least two different periods include aforward-feeding period and a reverse-feeding period, and the constantfeeding speed of the forward-feeding period is greater than a feedingspeed of the welding wire at the time of switching from the first periodto the second period.
 2. The method according to claim 1, furthercomprising increasing the feeding speed of the welding wire to aforward-feeding peak value after switching the first period to thesecond period.
 3. The method according to claim 1, wherein the secondperiod has a first cycle with a first forward-feeding peak value and asecond cycle with a second forward-feeding peak value, the second cycleis subsequent to the first cycle, and the first forward-feeding peakvalue differs from the second forward-feeding peak cycle.
 4. The methodaccording to claim 1, wherein the short-circuit transfer arc welding isswitched to the pulse arc welding at a timing when arcing reoccurs andthe welding current is at a low level during the short-circuit transferarc welding.
 5. An arc welding method comprising: performing pulse arcwelding with a welding wire being fed in a forward direction during afirst period; performing short-circuit transfer arc welding with thewelding wire being fed in the forward direction and a reverse directionduring a second period; and alternately switching between the firstperiod and the second period, wherein switching the first period to thesecond period is performed prior to a peak period of the short-circuittransfer arc welding and when a welding voltage and a welding currentreach peak values, the second period has at least two different periodsduring which the welding wire is fed at a constant feeding speed, and afeeding speed of the welding wire immediately increases at the time ofswitching from the first period to the second period.
 6. The methodaccording to claim 5, wherein the feeding speed of the welding wireimmediately increases towards a forwarding-feeding peak value at thetime of switching from the first period to the second period.
 7. Themethod according to claim 5, wherein the short-circuit transfer arcwelding is switched to the pulse arc welding at a time when arcingreoccurs and the welding current is at a low level during theshort-circuit transfer arc welding.